Mechanical orientation of magnetically anisotropic particles



United States Patent M 3,141,050 MECHANICAL ORIENTATION 0F MAGNETKCAL- LY ANISOTROPIC PARTICLES Walter S. Blume, Jr., Cincinnati, Ohio, assignor to Leyman Corporation, Cincinnati, Ohio, a corporation of Ohio No Drawing. Filed Aug. 16, 1960, Ser. No. 49,846 6 Claims. (Cl. 264-175) This invention relates to permanent magnets and is particularly directed to improvements in the manufacture of permanent magnets from magnetically anisotropic materials.

This application is a continuation-in-part of my copending application Serial No. 748,705, filed July 15, 1958, entitled Mechanical Orientation of Magnetically Anisotropic Particles, now Patent No. 2,999,275.

A principal objective of the present invention has been to provide an improved process for producing permanent magnets of excellent magnetic quality which are machinable by conventional shaping techniques.

Permanent magnets have long been produced from bulk metal alloys such as Alnico. Such magnets, however, are physically so hard that they cannot be cut except by grinding with abrasive wheels. For that reason the conventional mode of fabrication has been to cast the molten alloy composition in a mold conforming to the ultimate shape desired. Where dimensional accuracy is requisite, the casting is then ground to form or size. The cost of that mode of fabrication is obviously appreciable, the surface finish of the unground casting generally is poor, and there is considerable variation from piece to piece in all unground dimensions.

More recently, it has been learned that permanent magnets of improved quality can be made from so-called single domain or fine particle anisotropic materials. These materials are typified, for instance, by the ferrites of barium, lead, and strontium. By anisotropic is meant that such materials may be more easily magnetized in certain relative directions, called preferred magnetic directions, than in others. According to these findings, if, for example, barium ferrite is comminuted or ground to ultra-fine particles, of the order of, say 0.5 microns (.00005 cm.) in diameter, the particles behave as elementary magnetic units, called single magnetic domains. By aligning single domain particles of an anisotropic permanent magnet material so that the preferred magnetic axes of the particles are parallel, and by then immobilizing the particles in the oriented or aligned condition, an aggregate is formed which displays significantly better properties than were previously obtainable.

Heretofore, magnets of fine particle ferrite materials have usually been made by compressing the particles and then sintering them together at a high temperature, a technique which of necessity entails the use of expensive dies. In addition, the compressed powder structure, after removal from the die and before sintering, is extremely fragile and must be handled with great care. There is a high percentage of rejects. The sintered product is itself rather brittle. It cannot be machined, nor can it be subjected to rough usage without chipping at its edges. Furthermore, unless the sintering is performed very carefully, undue crystal growth occurs which reduces the coercivity of the magnet and thereby to a certain extent defeats the improvement of magnetic qualities which the technique is intended to provide.

Machinable fine particle magnets have been produced by molding mixtures of subdivided magnetic material and plastic, but no commercially practicable method of orienting domain sized magnetic particles in a plastic matrix has been available, and as a result the products of such processes do not possess the superior properties peculiar to aligned fine particle anisotropic materials and are inevitably of relatively poor magnetic quality. For example, it has been attempted to magnetically align the particles in the matrix so that the preferred directions of all the particles are parallel. The domains, being themselves elementary magnets, are acted upon and tend to be aligned by an externally applied magnetic field. Where the domains are physically compacted or are embedded in a viscous matrix, however, frictional stresses and/or the formation of interlocking dipoles between adjacent domains, as well as the general immobility of the domains in the matrix, tend to resist the orienting force of the magnetic field. Thus, while the field exerts a torque about the domains tending to align them, the torque is very small in relation to the interparticle forces and may orient the domains only to a small extent, if at all. For that reason, the degree of alignment accomplished by the application of an external field is at best small or partial, and the method is incapable of enabling the full magnetic potentialities of the anisotropic particles to be realized.

In my co-pending application S.N. 748,705 there is described and claimed a process for manufacturing permanent magnets from magnetically anisotropic materials which display properties comparable to or exceeding those of bulk magnets previously known and which also possess qualities of machinability, workability, and cutability making them amenable to fabrication by the use of ordinary cutting tools. The magnets are preferably made from particles of barium, strontium, or lead ferrite, or mixtures thereof, by reason of the low cost of those materials, but the methods of fabrication which the process provides may also be used in the preparation of machinable permanent magnets from various elements, compounds, or alloys such as particulate manganese-bismuth, elongated single domain iron particles, as well as other magnetically anisotropic particulate materials.

The essence of the process of that application lies in the concept of mechanically orienting or aligning the preferred magnetic axes of the domains with respect to each other in the matrix, rather than aligning them magnetically. Much better orientation of the particles can be achieved in that manner. Moreover, since a magnetic field is not utilized, the method makes possible the inexpensive production of permanent magnet material in large sizes.

According to the process of that application, the anisotropic material is ground to a suitable state of fineness, preferably but not necessarily domain size, and the particles are disposed in a workable non-magnetic, i.e. non-ferromagnetic, matrix medium such as rubber, polyethylene, plasticized polyvinyl chloride, or the like. The particles are made to disarrange themselves from a heterogeneous pattern of disorganization within the matrix into an orderly pattern of magnetic orientation and alignment by subjecting the composition to strong mechanical force in the nature of shearing stress such as is exerted internally and externally upon a mass as it passes through closely spaced rolls or an extrusion die. To illustrate one practice of the method of that application, by Way of example, the orientation may be conducted by adding domain-sized barium ferrite powder to a natural rubber base and milling the resulting composition into sheets by means of a conventional roll-type rubber mill wherein the composition is subjected to a shearing action by the rolls between which it is passed, preferably a number of times. The milling process disperses the magnetic material evenly throughout the rubber base, and as an incident thereto simultaneously orients the domains so that their preferred directional axes are parallel to one another.

When the ferrites of barium, strontium, and lead are comminuted to particles of domain size, the bulk compounds fracture into plate-like particles having two roughly parallel surfaces and an irregular edge. Peculiarly, the preferred direction of magnetization of the ferrite plates is normal to the two parallel surfaces; i.e., the domain plates are more easily magnetized if the magnetic lines of force of an applied external field are perpendicular to the plate. It is theorized that by the process of my copending application the plate-like particles are rotated within the matrix, as the composition is formed into a sheet, in such manner that the plane surfaces of the plates assume positions parallel to the surface of the sheet, the preferred magnetic axes of the plates being normal to the sheet surface.

The individual sheets so produced may be used to furnish permanent magnets, or a plurality of sheets may be stacked on top of each other and integrated to furnish a desired thickness from which magnets are then formed, after which the magnets are magnetized. The permanent magnets so fabricated are durable, easily machinable, and possess excellent magnetic qualities, comparable even with those of the Alnico alloys. They are inexpensive to produce since the raw materials are themselves inexpensive and since the process involves no unusually costly methods.

The present invention relates to improvements in the process of my co-pending application S.N. 748,705, and in particular to improvements in the method of preparing and sheeting the magnetic particle-matrix composition whereby particle orientation is obtained. It is predicated on the empirical determination that anisotropic permanently magnetic particles can be aligned in a nonmagnetic workable matrix by intermixing the magnetic particles with the matrix material to form a dispersion, subdividing or pulverizing the dispersion to form granules, and subjecting the granules to shear and/or compression forces such that they unite to form a sheet, particle alignment occurring as an incident to the sheeting procedure. Otherwise expressed, I have discovered that by pulverizing or granulating a matrix material which is loaded with anisotropic magnetic particles and sheeting a plurality of the granules so formed, a permanent magnet material is formed which displays excellent magnetic alignment as well as good physical properties.

According to a preferred embodiment of the invention, the magnetic particle-matrix intermixture is mechanically pulverized to form granules which are then subjected to shearing and/or compression forces between spaced counter-rotating rolls whereby the granules are united to form a homogeneous sheet in which the magnetic particles are aligned so that their preferred directions of magnetization are substantially parallel.

The process may be practiced with a variety of matrix materials as well as with various anisotropic permanent magnet materials.

In general, the matrix may be a workable, non-ferromagnetic solid or semi-solid material, such as rubber, resin, plastic, elastomer or metal, or may be a viscous liquid, in which the magnetic particles can be dispersed and which is capable of being granulated when loaded with the particles and then sheeted and hardened, set, or cured if necessary. According to one method, for example, the magnetic particles are dispersed in natural rubber, which, upon being granulated and sheeted, is cured to immobilize the particles within it; the application of heat to the mass after orientation cures or stabilizes the rubber to provide the desired coherence without disturbing the directional alignment of the particles. In other words, the matrix material may be any nonmagnetic material which may be granulated when loaded with the magnetic particles and reunited by the application of shear and/or compression forces thereon and which is sufficiently workable to transmit such forces to the particles yet plastic enough to permit the particles to move within it in response to those forces.

While the invention is particularly disclosed hereinafter in relation to the use of barium, lead, or strontium ferrite 4 on account of their low cost and abundancy, the method of orientation provided by the present invention is equally applicable to any anisotropic magnetic material particles of which are capable of being acted upon by internal mechanical stresses in a manner achieving orientation. The only limitation on the magnetic material, in other words, is that particles of it possess a preferred magnetic axis which lies consistently on a geometrically unique axis or axes such that the mechanical forces or turning moments acting upon the particles during the orienting step do not act in every direction with equal probability.

Description Barium ferrite is ground to particles of substantially domain size. This may be accomplished, for example, by grinding a commercial ferrite powder for about three hours in a standard Szegvari Attritor.

Uncured natural rubber, for example No. 1 Smoked Ribbed Sheet, is masticated preparatory to the addition of the magnetic powder by passing it through closely spaced rolls a number of times, so that it becomes softer and more workable.

The rubber matrix material and the magnetic powder are then intermixed, preferably in a standard Banbury mill or similar intensive mixer. Mixing is continued until the magnetic particles are dispersed substantially uniformly throughout the rubber, which may take about five minutes. The indicated temperature of the mixture should be kept below roughly 300 F. since the rubber may be adversely affected at high temperatures. Because of the speed and efficiency of its mixing action, use of a Banbury mill is preferred, but the particular method of distributing the magnetic particles throughout the matrix is not critical, and it is contemplated that other techniques can be utilized if desirable. In general, this step may be carried out in any manner which is effective to homogeneously distribute the magnetic powder throughout the particular matrix utilized.

The ratio of magnetic material to matrix material in the mixture is susceptible of considerable variation, but obviously the best magnetic qualities are obtained when the loading of the matrix material is as high as possible. However, the mechanical strength of the product is diminished if the quantity of matrix material is so small that it cannot adequately bind the magnetic particles together. A ratio of 16.76 pounds of barium ferrite to 1.295 pounds of natural rubber, for example, is suitable both from the standpoint of the magnetic qualities of the finished product and from the standpoint of its mechanical qualities. Wax and stearic acid may be added to this composition during the mixing step to aid respectively in mixing and in curing the rubber, preferably in the amount of 25.5 grams of wax and 6.5 grams of stearic acid.

The addition of the magnetic filler to the matrix causes the matrix to lose some of its workability and to become relatively stiffer and less coherent. If the loading is high, the composition actually tends to crumble during mixing. The quantity of magnetic material in admixture with a given matrix material which produces such crumbling of the composition depends at least in part on the size of the magnetic particles and on the nature of the matrix material. Natural rubber loaded with domain size ferrite particles begins to crumble when the ferrite particles comprise roughly half of the volume of the composition. Where the weights of those ingredients are as previously specified, the ferrite constitutes about 65% of .the volume of the mixture, and the mixture as it comes from a Banbury mill is in the form of bits and pieces of irregular size and shape. In general, it can be said that this tendency to crumble occurs when any binder or matrix is loaded with a filler-type ingredient to such an extent that the matrix cannot sufliciently wet the surfaces of the filler particles to bind them together. In accordance with this invention, as will be explained, by subdividing or granulating the magnetic particle-matrix intermixture and then subjecting the granules so formed to shear and/or compression forces, the granules are surprisingly formed into a cohesive, handleable sheet-like body and the magnetic particles simultaneously aligned within the body.

To this intermixture of magnetic particles and rubber matrix material are then added the conventional vulcanizing agents and accelerators. Where the proportions are as previously given, a suitable product may be obtained by adding to the mixture 3.2 grams of Methyl Tuads, 6.4 grams of sulfur and 12.9 grams of Altax (a vulcanization accelerator). By adding the accelerators after the mixing operation rather than during it, the rubber matrix is prevented from being cured by the considerable quantity of heat which is developed as the relatively large amount of magnetic filler is being added, so that the composition remains sheetable.

The intermixture is now subdivided to form granules. I have surprisingly found that by first pulverizing the intermixture to small granules, for example, which will pass a IO-mesh screen, the subsequent aligning and sheeting step is greatly expedited and facilitated. The size of the granules to which the intermixture is pulverized is not critical, nor is the method of granulation or subdivision. In general however, the smaller and more uniformly sized the granules are, the more readily they may be sheeted to produce a homogeneous, dense, smooth-grained sheet. (For that reason it is preferred to subject the mixture referred to in this example to such pulverizing even after it has been initially broken up during mixing in a Banbury mill.) A conventional micropulverizer works well in commercial practice, and quickly reduces the mixture to substantially uniform granules which will pass a ZO-mesh screen.

The granules each comprise a dispersion of magnetic particles randomly disposed in a bit of matrix material. The next step of the procedure, wherein the granules are subjected to shearing and/ or compression forces, is efiective to reconstitute, fuse, or blend the granules to form an extended body, i.e., a sheet, slab, or strip, and simultaneously to orient the magnetic particles in the matrix so that their preferred axes are substantially parallel to each other and are uniformly directed with respect to the extended body so formed.

In a preferred embodiment of the invention, this step is eifected by feeding the granules between closely spaced driven rolls. In a single passage between the rolls the rubber-magnetic granules are combined by the shear and/or compression forces acting on them such that a coherent, self-supporting, handleable sheet is formed which displays excellent particle alignment. For example, where the granules are of ZO-mesh size, the spacing between the rolls may be nominally about .030 inch; an extended sheet is formed which is about .080 inch thick. The procedure is effective over a range of roll spacings and sheet thicknesses, and good alignment can, in fact, be obtained by sheeting granules of coarser size between more widely spaced rolls to thicknesses of and more.

While it is perhaps most convenient to sheet the granules between rolls, particle orientation may be achieved by other procedures wherein shear and/or compression forces are exerted on the granules tending to reduce them in thickness. As is explained in my co-pending application Serial No. 748,705, it is theorized that the effect of these forces is to tip over the ferrite plates so that the (roughly) parallel surfaces of the plates are arranged more or less parallel to the surface of the extended body. Thus, where the forces are applied by passing the granules between rolls, the reduction in thickness as the granules pass between the rolls produces stresses within the granules which act upon the magnetic particles to align them. At the same time these forces, together with accompanying heat and/or pressure, fuse the physically discrete granules into a coherent body.

The rolls utilized in the preferred method described may be differentially speeded; if desired, a ratio of 1.1:1 is suitable.

While substantial particle alignment is effected in a single pass between the rolls, the degree of alignment can often be improved by thereafter rolling the sheet in such manner as to reduce its thickness. Thus, if the rolls are relatively widely spaced when the granules are sheeted, the shear forces may not be transmitted to the interior of the granules or sheet, so that the magnetic particles in that region are not aligned. Reducing the thickness of the sheet in further passes between rolls is eifective to align these particles. In general, the greater the relative reduction in thickness of the sheet as it is passed between the rolls, the greater the alignment obtained per pass.

A plurality of sheets so formed can be laminated to form still thicker sheets, as has been described in my co-pending application S.N. 748,705. This lamination process does not aifect the overall alignment of the particles since the preferred directions of magnetization of the particles are parallel when the sheets are stacked in facial juxtaposition. Lamination to build up the thickness of the composition is readily effected, for example, by passing two or more sheets which are in facial contact between rolls to integrate and adhere them. Sheets or blocks of such thicknesses as desired may be built up in this manner.

Heat is developed in the sheet under the forces acting on it during the aligning procedure. Where the matrix is rubber, this heat tends to soften the matrix so that it becomes more workable, but the heat should not be allowed to become so high as to cause premature vulcanization of the rubber. About l25-150 F. is a good operating temperature range in which to sheet a composition having the rubber matrix described.

The sheet or laminate is then cured by conventional vulcanization technique. A vulcanization period of about 12 minutes at about 278 F. is suitable. This step may be omitted or adjusted appropriately to cure, harden or set other matrix materials.

Magnets of any desired configuration may be cut from the sheet or laminate. The orientation of the particles is not disturbed during the forming of the magnet because the magnetic particles are firmly immobilized in the matrix. The product so formed is magnetized by placing it in a magnetic field extending parallel to its preferred direction of magnetization.

Strontium or lead ferrite may be substituted for, or mixed with, the barium ferrite disclosed in the detailed description. Orientation is obtained with equal facility where the magnetic particles are elongated, e.g. single domain iron particles, although here it will be understood that the preferred direction of magnetization of the particles is longitudinal and consequently the preferred axis of the sheeted composition will be in the plane of the sheet rather than normal to it.

Having described my invention, I claim:

1. In the process of making a permanent magnet material by mixing a rubber binder with a very high proportion of ultrafine, substantially single domain size particles of a permanent magnet material and rolling the mixture to form the same into a coherent elongated body which is reduced in thickness relative to said mixture, wherein the said particles comprise at least about of the mixture by weight, the method of rendering said mixture more readily sheetable which comprises, subdividing said mixture into granules before said rolling, said granules being at least small enough to pass a 10 mesh screen, each of said granules comprising a mixture of said particles and said rubber binder, and then subjecting a plurality of the granules, in the solid state, to said rolling, said granules thereby forming a coherent elongated body more readily by said rolling than will the mixture in the absence of such subdivision.

2. A method of improving the sheeting characteristics of a highly loaded mixture of rubber and ultrafine, hard, refractory particles of a permanent magnet substance of the ferrite type, wherein the weight ratio of the magnetic particles to the rubber in said mixture is at least about 8:1 and also wherein the magnetic particles comprise between 50% and 70% of the volume of the mixture, said method comprising, subdividing said mixture into granules small enough to pass a 10 mesh screen, and thereafter forming a plurality of said granules without liquification thereof into an elongated body by subjecting said granules to pressure and mechanical shear forces between a pair of rolls, said granules thereby being formed into a coherent, substantially uncracked uniform body more readily than said mixture can be in the absence of such subdivision.

3. A method of improving the sheeting characteristics of a mixture of uncured rubber and ultrafine particles of substantially domain size barium ferrite, wherein the ferrite particles comprise about 55-70% of the total volume of the mixture, said method comprising, subdividing said mixture into granules small enough to pass a 20 mesh screen, and thereafter forming a plurality of said granules in the solid state into a sheet by rolling said granules under pressure between rolls, said granules thereby being formed into a coherent uniform continuous sheet more readily than said mixture can be in the absence of such subdivision.

4. The method of claim 3 wherein said granules are formed into a sheet between said rolls at a temperature of about 125-150" F.

5. The method of claim 3 wherein said rubber is uncured natural rubber.

6. A method of improving the sheeting characteristics of a mixture of uncured rubber and substantially domain size, hard, refractory particles of a permanent magnetic substance of the class consisting of the ferrites of barium, strontium and lead, wherein the weight ratio of the magnetic particles to the elastomer in said mixture is at least about 8:1 and also wherein the particles comprise between about and of the volume of the mixture, said method comprising, subdividing said mixture into granules small enough to pass a 10 mesh screen and thereafter forming a plurality of said granules in solid form into a uniform elongated body by subjecting said granules to shear and compression pressure forces between rolls, said granules thereby being formed into a uniform elongated body substantially more rapidly than the unsubdivided mixture can be, and thereafter vulcanizing the body so formed.

References Cited in the file of this patent UNITED STATES PATENTS 1,687,441 Grosjean Oct. 9, 1928 1,700,208 Parsseau Jan. 29, 1929 1,763,314 McConoughey June 10, 1930 2,314,062 Alvey et a1 Mar. 16, 1943 2,461,089 Smidth Feb. 8, 1949 2,464,746 Gering Mar. 15, 1949 2,620,320 Novak et al Dec. 2, 1952 2,746,086 Vickers May 22, 1956 2,758,336 Franssen Aug. 14, 1956 2,811,750 Cofek Nov. 5, 1957 2,849,312 Peterman Aug. 26, 1958 2,882,554 Heck Apr. 21, 1959 2,999,275 Blum Sept. 12, 1961 3,012,282 Donald Dec. 12, 1961 FOREIGN PATENTS 565,556 Belgium Mar. 31, 1958 

3. A METHOD OF IMPROVING THE SHEETING CHARACTERISTICS OF MIXTURE OF UNCURED RUBBER AND ULTRAFINE PARTICLES OF SUBSTANTIALLY DOMAIN SIZE BARIUM FERRITE WHEREIN THE FERRITE PARTICLES COMPRISE ABOUT 55-70% OF THE TOTAL VOLUME OF THE MIXTURE, SAID METHOD COMPRISING SUBDIVIDING SAID MIXTURE INTO GRANULES SMALL ENOUGH TO PASS A 20 MESH SCREEN, AND THEREAFTER FORMING A PLURALITY OF SAID GRANULES IN THE SOLID STATE INTO A SHEET BY ROLLING SAID GRANULES UNDER PRESSURE BETWEEN ROLLS, SAID GRANULES THEREBY BEING FORMED INTO A COHERENT UNIFORM CONTINUOUS SHEET MORE READILY THAN SAID MIXTURE CAN BE IN THE ABSENCE OF SUCH SUBDIVISION. 